1. Field of the Invention
The present invention is related to a current limit control method of DC motor and related device and motor control circuit, and more particularly, to a current limit control method of DC motor and related device and motor control circuit which can prevent the influence from the circuit components and the environmental factors and perform high degree of accuracy of measurement to the motor current.
2. Description of the Prior Art
A DC motor has very wide application in our everyday life. For example, a fan driven by the DC motor can be installed on a CPU chip to help dissipating heat of the corresponding personal computer. Also, inside every optical disk drive or hard disk drive, the DC motor is used to drive the optical disk or the hard disk. On the other hand, applications like robots and toys also use DC motors for various purposes. Generally speaking, the working principle of the DC brushless motor is to conduct a current through the stator coil of the DC motor to generate a magnetic field; the magnetic field generated by the stator is then interacted with the magnetic field of the rotor (armature), so the rotor can start rotating, and the DC motor can be considered as a device which converts electrical energy into mechanical torque.
The DC motor is usually accompanied with a controller which is use to control and drive the motor. Please refer to FIG. 1, which illustrates a schematic diagram of a DC motor circuit 10 according to the prior art. The DC motor circuit 10 comprises a power supply device PY1, power control switches PSW1˜PSW4, a comparator COMP, a controller CNTL and a DC motor MOTOR. According to FIG. 1, the DC motor MOTOR is represented by an inductor, and OUTA1 and OUTB1 are the two endpoints of the motor MOTOR. Usually, the power control switches PSW1 and PSW3, which are connected between the power supply device PY1 and the DC motor MOTOR, are also called the upper gate switches; on the other hand, the power control switches PSW2 and PSW4, connected between the DC motor MOTOR and the ground end, are also called the lower gate switches. Besides that, the DC motor circuit 10 also comprises two bypass capacitors CVM1 and CVCC1 for stabilizing the power voltage level, and a detecting resistor RS1 is for detecting the motor current. Also, according to FIG. 1, the two endpoints of the detecting resistor RS1 are VCC1 and VM1, respectively, and by detecting the voltage drop between the two endpoints VCC1 and VM1 of the detecting resistor RS1, the magnitude of the motor current can be determined. On the other hand, the controller CNTL can limit the magnitude of the motor current by controlling the “ON” and “OFF” actions of the power control switches PSW1˜PSW4. This is extremely important because, for certain circumstances, a large motor current could damage the circuit, the components and the motor itself.
While in the state of normal operation, the motor can be operated by switching between two states: the first motor driving state and the second motor driving state. On the first motor driving state, the controller CNTL turns on the upper bridge switch PSW1 and the corresponding lower gate switch PSW2, such that a current can be conducted from the power supply device PY1, through the power control switch PSW1 to the DC motor MOTOR; after that, the current is then directed through the power control switch PSW2 to the ground, such that the energy can be transferred from the power supply device PY1 to the DC motor MOTOR. On the second motor driving state, the controller CNTL can turn on the upper bridge switch PSW3 and the corresponding lower gate switch PSW4, and the current is conducted from the power supply device PY1, through the power control switch PSW3 to the DC motor MOTOR, and then through the power control switch PSW4, and directed to the ground, and the energy can also be transferred to the DC motor MOTOR. By switching between the first motor driving state and the second motor driving state, the controller CNTL can keep the motor running (please note that the above description gives an example for single-phase motor type only, the operations of multiple phase motor can also be derived by analogy). However, if the magnitude of the current flowing through the DC motor MOTOR is greater than a current limit value ILIM1 (which is determined by the designer according to the functional characteristics or the electrical maximum-rating of device of motor or other related components) it is possible that the motor circuit will be damaged by the current; to prevent this to be happened, a detecting circuit is required to constantly detect the magnitude of the motor current, and then the controller CNTL can temporarily shut down the current (energy) supply path from the power supply device PY1 to the DC motor MOTOR, by temporarily turning off the upper gate switch PSW1 (corresponding to the first motor driving state) or PSW3 (corresponding to the second motor driving state) when the motor current surpasses a current limit value ILIM1, so the motor current can be controlled under the current limit value ILIM1.
Please continue to refer to FIG. 1. Inside the figure, the comparator COMP detects the voltage difference across the ends of the detecting resistor RS1 to indirectly measure the magnitude of the current flowing through the DC motor MOTOR. However, the DC motor circuit 10 has several drawbacks. First, the detecting resistor RS1 in the DC motor circuit 10 must be a high-power precision resistor, and the cost is high. Next, when the upper gate switch PSW1 or PSW3 turns on (or turns off), for a short moment, the bypass capacitors CVM and CVCC, which were originally used for power voltage stabilization, can provide a current pulse to the DC motor MOTOR, such that the current flowing through the DC motor may be very different from the current flowing through the detecting resistor RS1. In other words, when the upper gate switch PSW1 or PSW3 just turns on (or turns off), the current flowing through the DC motor MOTOR may include both the current from the capacitor CVM and the current flowing through the detecting resistor RS1; therefore, for example, at the moment when switch PSW1 or PSW3 just turns on (or turns off), if the measurement of the current flowing through the detecting resistor has shown that it is 1.0 Amp, the current flowing through the DC motor MOTOR might actually be about 1.5 Amp or even larger. Besides that, since the function of capacitor CVM is mainly for voltage stabilization, very often the capacitance of the capacitor CVM is large, therefore when the upper gate switch PSW1 or PSW3 is turned on, the capacitor can provide a current pulse of considerable magnitude to interfere the correct measurement of the real motor current. Also, the current flowing through the DC motor MOTOR mostly flows through the detecting resistor RS1, and thus converts plenty of electric power into thermal heat. On the other hand, if a resistor of smaller resistance is used for the detecting resistor RS1, the voltage drop across the detecting resistor RS1 will definitely become smaller; this will make the measurement of the current even more difficult and the accuracy becomes poorer. In this case, if the same degree of accuracy is to be achieved, the highly sensitive comparator COMP should be selected and/or some extra supporting circuitry should be added. Just for the same reason, only a few off-the-shelf resistors are proper to be utilized as the detecting resistor RS1, and many limitations are still restricting the operating range of the motor current and the precision of measurement.
Please refer to FIG. 2, which illustrates a schematic diagram of another DC motor circuit 20 according to the prior art. The DC motor circuit 20 is similar to the DC motor circuit 10 in many ways, and the components of the same function are using the same symbols for easy comparison. The DC motor circuit 20 utilizes a detecting resistor RS2, resistors RD1 and RD2, and a comparator COMP to detect the magnitude of the current flowing through the DC motor, and there is a controller 204 being used to control the DC motor MOTOR. By carefully examining the differences between the DC motor circuit 20 and the DC motor circuit 10, it can be observed that the detecting resistor RS2 for detecting the motor current is now placed between the lower gate switches (PSW2 and PSW4) and the ground end, instead of being placed close to the power supply device PY1. Owing to this major difference, the current flowing through the DC motor MOTOR will all flow to the ground through the detecting resistor RS2, and the DC motor circuit 20 can then detect the voltage drop between the ends of the detecting resistor RS2 to determine the magnitude of the magnitude of the motor current. On the other hand, connected to the negative end of the comparator COMP is a reference voltage VLIM1, which is output by the circuit composed of a voltage source VSTD1, and two resistors RD1 and RD2 as the voltage divider. By utilizing the comparator COMP to compare the voltage of an endpoint VSS1 with the reference voltage VLIM1, when the voltage VSS1 is greater than the reference voltage VLIM1, the comparator COMP will switch its output state and notify the controller 204 to turn off the upper gate switch PSW1 or PSW3 (depends on whether the DC motor circuit 20 is on the first motor driving state or the second motor driving state), such that the motor current and the rotational speed of the motor can be under control.
Inside the DC motor circuit 20, by placing the detecting resistor RS2 between the DC motor MOTOR and the ground end, the bypass capacitor can no longer interfere with the measurement of the motor current. However, the detecting resistor still has to be a high-power precision resistor, and the cost is still high, and plenty of electric power can still be wasted. Next, similar to the situations of the DC motor circuit 10, if a resistor of less resistance is being used for the detecting resistor RS2, even though less power will be wasted, but the voltage drop between the two ends of the detecting resistor RS2 will become too small to detect nice and easy, some more accessory circuit should be added to properly increase the precision of the measurement. Because of this, the range of the current limit will become very limited and the degree of precision of the motor current measurement is reduced. For example, if the resistance of the detecting resistor RS2 is 0.5Ω, and value of the current limit is set at 0.5 Amp, then the resistors RD1 and RD2 can be selected such that the reference voltage VLIM1 can be equal to 0.25 Volt (=0.5Ω*0.5 Amp). When the voltage drop between the two ends of the detecting resistor RS2 becomes equal to or greater than 0.25 Volt, the comparator COMP will switch its output state (by switching its output voltage), and turns off the upper gate switch PSW1 or PSW3 via the controller 204 to limit the motor current. On the other hand, if the current limit is increased to 1.0 Amp, the comparator COMP will change its output state when the voltage drop between the two ends of the detecting resistor RS2 becomes equal to or greater than 0.5 Volt. For the example above, when the current limit is increased from 0.5 Amp to 1.0 Amp, the voltage drop between the two ends of the detecting resistor RS2 will only slightly increase from 0.25 Volt to 0.5 Volt, as can been seen, the variation of voltage drop is small even though motor current had been doubled, so such method will still be improper for high precision measurement. To make a short summary, inside the DC motor circuit 20, because the resistance of the detecting resistor RS2 has to be a high-power precision resistor of very small resistance, when the value of the current limit changes, this requires the comparator COMP to be sensitive enough to detect the slight difference between the two ends of the detecting resistor RS2. And, the performance requirements of the comparator are pretty high and the cost of the component is relatively high.
However, differing from the lower gate switches PSW2 and PSW4, which are built in an integrated circuit (IC), the detecting resistor RS2 is often placed on a printed circuit board next to the integrated circuit, so the current limit can be adjusted more conveniently by different users or functions; unfortunately, a parasitic inductor can be introduced between the lower gate switches (PSW2 and/or PSW4) and the ground end. When the upper gate switch PSW1 or PSW3 suddenly turns on or off, this parasitic inductor can produce a voltage pulse on the end of the detecting resistor RS2, such that the comparator COMP can make wrong judgment. For example, when operated in the first motor driving state, the current is going from the power supply device PY1, through the power control switch PSW1 to drive the DC motor MOTOR, and then passing through the power control switch PSW2, and directed to the ground after passing the detecting resistor RS2. But, when the upper gate switch PSW1 suddenly turns off, because the motor current needs to keep its continuity (due to motor has inductance character, so the current still needs to go from the endpoint OUTA1, through the DC motor MOTOR, and to the endpoint OUTB1), a conducting loop was formed by having the current passing through the two lower gate switches PSW2 and PSW4, such that the motor current won't be stopped. Meanwhile, since the two lower gate switches formed a conducting current loop, the motor current will no longer flow to the ground GND. Noteworthily, even if the lower gate switch PSW4 is in an “OFF” state (being turned off by the controller 204), a current can still be conducted from the source of the lower gate switch PSW4, to the endpoint OUTB1 via the body diode of the lower gate switch PSW4, and the conducting loop can still be established. However, changing the path of the motor current will bring an abrupt change to the magnitude of the current flowing through the parasitic inductor. According to the operating principles of the inductor, when the current of the inductor experiences a sudden change, the voltage across the inductor will display a large voltage pulse. Therefore, when the parasitic inductor between the lower gate switches PSW2, PSW4 and the detecting resistor RS2 suddenly loses its current, across the ends of the parasitic inductor, a large voltage pulse will be generated, and the comparator COMP will make wrong judgment. To avoid this, the DC motor circuit 20 needs some extra circuit to avoid the misjudgment. Besides that, when the power control switches were turned on, the power control switches can be observed as an ideal resistor of a small resistance, and the resistance is inversely proportional to the “ON” voltage of the power control switch. In other words, the resistance of the power control switch is inversely proportional to the voltage difference between the gate and the source of the power control switch. Since there is a voltage drop across the ends of the detecting resistor RS2, the “ON” voltage will be lowered than the desired value, such that the resistance of a conducting power control switches PSW1˜PSW4 will be increased, and the conductivity of the power control switches PSW1˜PSW4 will be decreased; meanwhile, the power consumption will be increased, and the power efficiency will be decreased.